Conventionally, peripheral processing of a metal rod or a metal pipe has used the forging forming by plastic deformation using a punch or a die (see Patent Literatures 1 and 2 for example). Patent Literature 1 has proposed a method for press forming a can body having ribs on its outer surface or inner surface. In the proposed method, a female mold having an approximately circular cross section is press formed, a plurality of grooves are formed on the inner circumferential surface of the female mold or on the outer circumferential surface of a columnar punch, and pellets are put on the bottom of the female mold, and then, the punch is pressed against the pellet in the female mold to obtain a can body.
In addition, Patent Literature 2 has proposed a manufacturing method of pipe-shaped workpieces by forging as a method for forming a stepped part (serration part) on the bottom of the pipe-shaped material alternative to the conventional cutting method. In the manufacturing method described in Patent Literature 2, the bottom of the pipe-shaped material is guided by an upper mold into the annular gap formed between a guide pin and a serration-shaped concave-convex part, and the serration part is forging-formed on the outer circumference of the bottom part of the pipe-shaped material.
Forging forming by plastic deformation has also been applied as a technique for machining the outer circumference of a metal plate material in addition to metal rods or metal pipes. For example, Patent Literature 3 discloses a technique for forming a concave groove which technique can form a concave groove having a desired shape and a thin-thickness hard wall on the outer circumference of a metal plate material at low cost by press machining.
Further, as another method for machining the periphery of metals, Patent Literature 4 discloses a method for forming locking projections. The disclosed method presses plural circumferential positions around the holding hole of the mounting plate using a wedge-shaped first punch to form the primal-form of each locking projection by plastic deformation; and then each of the primal-form of locking projections is plastically deformed radially inwardly to form each locking projection.
Each of the above-mentioned machining methods is mainly for performing periphery processing of a metal rod or a metal pipe. On the other hand, however, various methods have also been proposed for machining the cross section of the end of a metal plate or a metal rod. For example, Patent Literature 5 has disclosed a method for producing integrally the flange part and the dust cover in manufacture of drum brakes. In the disclosed method, the outer circumference of a bottomed metal cylinder is cut using a cutting die while rotating the bottomed metal cylinder, and then drawing process is applied on the cut-part using a pressing die to form a cylindrical part that becomes the dust cover.
The method for splitting the end of metal described in Patent Literature 5 is for machining an edge part of periphery of a brake shoe that is a circular metal material. Other than a circular metal material, however, if there is a method that is able to divide, in the horizontal direction, the edge of a metal plate having any shape of rectangle, polygon, and ellipse, or is able to divide a metal rod having any of circular, elliptic, rectangular, and polygonal cross section, such splitting method sees expectation for various use. With this, the present invention previously developed a new method for splitting the end of a metal plate or a metal rod (Patent Literatures 4 and 5). In this new method for splitting the end part, the press-splitting with a slitting punch or a cleaving punch is performed not only one time but performed continuously many times, and the position of clamping dies, which pinch the metal plate or the metal rod, is moved at every press-splitting operation step so that the length of cleft created by splitting can be freely adjusted within a desired range to optimize.